Myocphenolate mofetil (MMF) and tacrolimus are the main immune suppressants used in over 90% of kidney transplants (tx). Several studies have shown an association between low mycophenolic acid (MPA, active metabolite of MMF) exposure as measured by 12 hour area-under-the-curve (AUC) and acute allograft rejection. However, significant inter-individual variation in the pharmacokinetics (PK) of MMF exists with as much as a 10-fold variation in AUC with the same dose. Elevated MPA concentrations have been associated in some studies with toxicities such as diarrhea, leukopenia and anemia. PK studies show a second peak of MPA 6-8 hours after oral MMF from enterohepatic recycling (EHR) of MPA due to biliary excretion of its phenolic glucuronide (MPAG, major inactive metabolite of MPA). EHR occurs by hydrolysis of the glucuronide by ?- glucuronidases produced by gut bacteria and reabsorption of MPA. Extensive EHR greatly increases systemic exposure to MPA and likely enhances immunosuppression and toxicity. ?-glucuronidases are produced by the gut microbes Streptococcus agalactiae, Clostridium perfringens, and E. coli and are known to influence drug substrates and potentially the extent of EHR. In a pilot study of stool from tx patients, Bacteroides, Ruminococcus, Coprococcus, and Dorea were significantly lower in tx patients with diarrhea. The specific hypothesis is that certain patterns of stool microbiota lead to increased EHR, greater MPA systemic exposure and toxicities. Therefore, we will determine the association of microbiome with: MPA and metabolite PK posttx and EHR (Aim 1) and MPA associated toxicities such as anemia and leukopenia posttx (Aim 2). Lastly we will determine the association of microbiome diversity with diarrhea in tx recipients on MPA posttx (Aim 3). A prospective cohort of new kidney txs with serial stool microbiome samples will be developed and MPA and metabolite PK and EHR measured (Aim 1), and subsequent assessment of leukopenia and anemia (Aim 2). We will measure serial stool microbiome for association with diarrhea posttx (Aim 3). All subjects will be followed for 12 months posttx for estimated glomerular filtration rate which is associated with long-term kidney outcomes. The microbiome will be determined using innovative, shotgun sequencing for all aims, along with Internal Transcribed Spacer 2 (ITS2) gene sequencing for fungi for Aim 3. We will use metatranscriptomics of microbial ?-glucuronidases, using RNAseq, to elucidate the mechanism through which the microbiome influences MPA exposure, and MPA associated leukopenia and anemia, and posttx diarrhea. We will collect patient centered outcomes of diarrhea using text messaging. By the end of the study, we will have developed microbiota patterns that could be utilized as a predictive test for when MPA PK assessment is needed, for selection of MPA dose to minimize toxicities. This study can lead to future use of select antibiotics or fecal transplants to restore the microbiome thereby assuring normalized drug disposition and minimal MPA related toxicities. We will have created a model to study other drugs that undergo EHR or which have perturbed absorption or distribution due to the microbiome.